Make build-config magic use memfd by default.

crates/runtime/src/instance/allocator/pooling.rs

@@ -703,7 +703,7 @@ impl MemoryPool {
         let mapping = Mmap::accessible_reserved(0, allocation_size)
             .context("failed to create memory pool mapping")?;
 
-        let num_memfd_slots = if cfg!(feature = "memfd-allocator") {
+        let num_memfd_slots = if cfg!(memfd) {
             max_instances * max_memories
         } else {
             0

crates/runtime/src/lib.rs

@@ -19,7 +19,7 @@
         clippy::use_self
     )
 )]
-#![cfg_attr(feature = "memfd-allocator", allow(dead_code))]
+#![cfg_attr(memfd, allow(dead_code))]
 
 use std::sync::atomic::AtomicU64;
 
@@ -67,14 +67,14 @@ pub use crate::vmcontext::{
 mod module_id;
 pub use module_id::{CompiledModuleId, CompiledModuleIdAllocator};
 
-#[cfg(feature = "memfd-allocator")]
+#[cfg(memfd)]
 mod memfd;
-#[cfg(feature = "memfd-allocator")]
+#[cfg(memfd)]
 pub use crate::memfd::{MemFdSlot, MemoryMemFd, ModuleMemFds};
 
-#[cfg(not(feature = "memfd-allocator"))]
+#[cfg(not(memfd))]
 mod memfd_disabled;
-#[cfg(not(feature = "memfd-allocator"))]
+#[cfg(not(memfd))]
 pub use crate::memfd_disabled::{MemFdSlot, MemoryMemFd, ModuleMemFds};
 
 /// Version number of this crate.

crates/runtime/src/memfd_disabled.rs

@@ -37,11 +37,9 @@ impl ModuleMemFds {
 /// To allow MemFdSlot to be unconditionally passed around in various
 /// places (e.g. a `Memory`), we define a zero-sized type when memfd is
 /// not included in the build.
-#[cfg(not(feature = "memfd-allocator"))]
 #[derive(Debug)]
 pub struct MemFdSlot;
 
-#[cfg(not(feature = "memfd-allocator"))]
 #[allow(dead_code)]
 impl MemFdSlot {
     pub(crate) fn create(_: *mut libc::c_void, _: usize) -> Self {

crates/runtime/src/traphandlers/unix.rs

@@ -54,14 +54,7 @@ pub unsafe fn platform_init() {
     // Sometimes we need to handle SIGBUS too:
     // - On ARM, handle Unaligned Accesses.
     // - On Darwin, guard page accesses are raised as SIGBUS.
-    // - With the MemFD allocator, heap growth is controlled by
-    //   ftruncate'ing an mmap'd file, and so out-of-bounds accesses
-    //   are raised as SIGBUS.
-    if cfg!(target_arch = "arm")
-        || cfg!(target_os = "macos")
-        || cfg!(target_os = "freebsd")
-        || cfg!(feature = "memfd-allocator")
-    {
+    if cfg!(target_arch = "arm") || cfg!(target_os = "macos") || cfg!(target_os = "freebsd") {
         register(&mut PREV_SIGBUS, libc::SIGBUS);
     }
 }